This came briefly into use at the end of the eighteenth century. It is not an older form of the contraction, though, as shan't predates it in print by about a century. (Source: World Wide Words )
However, since it was written as "sha'n't" in as early as 1697 - albeit using the long-s - the reliability of the World Wide Words' statement is questionable. (Source: The Provok'd Wife: a Comedy, page 33 )